1. Field of the Invention
This invention relates to a projection lens and more particularly to a color corrected projection lens for use in a video projector which produces a large picture by projecting on a screen an image appearing on a cathode-ray tube (CRT).
2. Description of the Prior Art
Conventional projection lenses for projecting an image appearing on a CRT of a television receiver onto a screen are composed entirely of glass elements or composed entirely of plastic elements. Among these, one type comprise three units consisting of three elements having aspheric surfaces as disclosed, for example, in the Japanese Laid-open Patent Application No. 57-108818, and another type comprises three units consisting of four elements as in the Japanese Laid-open Patent Application No. 58-198017. When made entirely of plastic, the focal point of the lense changes significantly depending on the atmospheric temperature changes. Therefore, hybrid projection lenses composed of both glass elements and plastic elements have been proposed. Among these, one comprising three units consisting of three elements is known from, for example, the Japanese Laid-open Patent Application No. 58-125007, and another comprising three units consisting of four elements is disclosed, for example, in the Japanese Laid-open Patent Application No. 59-155818.
Generally, in a projection television using three CRTs, correction of chromatic aberration has not been required so much because CRT images of monochromatic colors of green, red and blue were magnified by the projection lens. However, the luminous spectrum of a phosphor of a CRT is spread, or is not strictly monochromatic. Compared with the luminous spectrum of a red light emitting phosphor, that of a blue or green light emitting phosphor spreads over a wider wavelength region.
Recently projection televisions have become used as information display terminals for computers. In this case, because of the necessity to magnify and project small character information, the projection lens is required to have greater resolution and clarity. Even if the monochromatic aberration is corrected and reduced, sufficient improvement of performance is not obtained. To solve this problem, a projection lens intended to correct the chromatic aberration is known. An all-plastic lens construction comprising four units consisting of four elements using aspheric surfaces is known from, for example, the Japanese Laid-open Patent Application No. 59-170812. A hybrid type of lens construction comprising four units consisting of five elements is known from the Japanese Laid-open Patent Application No. 58-139111.
In the conventional color corrected projection lenses, however, in the all-plastic lens construction, since the focal point is shifted significantly due to ambient temperature changes, a favorable picture quality can not be maintained, whereas in the hybrid construction which is stable against temperature changes, a bright projected image can not be obtained because the aperture ratio is as small as 1:1.3 to 1:1.4. In both constructions, moreover, since the half angle of view is as narrow as 20.degree. to 23.degree., it is necessary to extend the projection distance, that is, the distance from the screen to the projection lens in order to obtain a large image, which is a bottleneck for reducing the size of the projection television set.